As indicated by the Federal Communications Commission (FCC), the main cause of depletion of frequency resources for wireless communication is an inefficient frequency allocation policy (see Non-Patent Literature 1 below). Most of the frequency resources are allocated to a particular service provider with an exclusive license, and a strict constraint is imposed on the transmission power on the frequency resources in order to prevent a mutual interference. Under such conditions, a cognitive radio that is a wireless communication device capable of adaptively changing an operation parameter has been developed. With the advent of the cognitive radio, a new policy has been developed in many countries in order to open frequency resources within the range of not causing harmful interference with an unlicensed user.
A system of a service provider that has a license for a certain frequency channel is referred to as a primary system (PS). On the other hand, a system that secondarily uses the frequency channel protected for the sake of the primary system is referred to as a secondary system (SS). Approaches to the secondary use of the frequency channel are classified into two groups. The first approach is an opportunistic spectrum access that monitors the frequency resource of the primary system to detect the absence of a PS user and allows an SS user to use a so-called spectrum hole that has been detected (see Non-Patent Literature 2 below). The second approach is spectrum sharing that recognizes the coexistence of a PS user and an SS user and controls parameters such as the transmission power of the secondary system in order to prevent the SS user from giving interference exceeding an allowable level to the PS user (see Non-Patent Literature 3 below). Every approach is important in terms of the efficient use of frequency resources.